1. Field of the Invention
The present invention relates to test and measurement systems. More specifically, the present invention relates to high speed IF (intermediate frequency) power detectors.
While the present invention is described herein with reference to a preferred embodiment in an illustrative application, it is to be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings of the present invention will recognize additional modifications, embodiments and applications within the scope thereof.
2. Description of the Related Art
The high cost associated with the manufacture and launch of satellite systems and the subsequent inaccessibility thereof makes it imperative that the satellite be designed in all respects to provide reliable performance throughout its useful life. As a key component of the system, the antenna and its design must be proven to be satisfactory prior to launch. Accordingly, antenna test and measurement systems are used to fully exercise the antennas in many modes of operation. Some systems test the far field performance of the antenna. These systems are typically open loop. Other systems test the near field performance of antenna. The near field systems are typically closed loop in that the activation of the transmit antenna and the collection of data from the antenna under test are under the control of a single system controller. The far field tests are typically concerned with signal strength or amplitude. The near field tests typically require amplitude as well as phase information to extract the far field radiation pattern.
The tests are typically quite extensive. Transmit and/or receive beam characteristics are often tested as a function of many variables, parameters and operating conditions. The gain, for example, may be measured as a function of frequency, azimuth, elevation, distance and/or time. Numerous combinations and permutations of variables may be used as part of a single testing program.
In addition, where the antenna is of the phased array variety having a plurality of sensing and/or receiving elements (feeds), beam forming networks, input and output channels and/or input and output ports, the test may involve switching various combinations of feeds through various combinations of channels and ports. It is often desirable to run such tests using any of several signal polarization states.
It is not difficult then to imagine how in some tests, as many as ten million gain measurements alone may be required. This is typically a time consuming process requiring specialized equipment and personnel skilled in the field. As such, the testing of the antenna adds significantly to the development and manufacturing cost of the overall system. There is therefore a generally recognized need in the art to minimize the time required to completely test such antennas.
There is a countervailing need to perform more and more tests as antennas become more capable and complex. For example, while the antenna scans in azimuth, it was typically necessary to make a gain measurement at certain ports, channels and feeds at a specific frequency. However, there has now been recognized a need to make such measurements while simultaneously hopping in frequency.
Conventional measurement systems have had difficulty meeting the demanding requirements of this application. The operating speed of conventional systems has been a primary limitation. As discussed more fully below, limitations on the speed of conventional systems may result from the scheme used to detect and measure input power or gain and/or the capability of the system to switch from one set of ports to another.
Many prior art systems also require a computer to control the complex system through the testing program. In such systems, the computer is required to set up and energize the antennas, switch channels in and out of the testing apparatus, make and process the desired measurements, and store the resulting data. It is generally desirable to free the computer from such tasks. This would allow for additional increases in system speed and/or operating capability.
To make the above-noted antenna gain measurements, conventional systems typically use one of the following detection techniques: analog non-linear circuits such as square law devices and half-wave and full-wave rectifiers, analog-to-digital conversion with subsequent digital processing to accomplish detection, analog synchronous detectors, detector log video amplifiers, thermal (caloric) conversion circuits, and circuits employing conventional analog computational functions such as true RMS to DC converters and DC log amplifiers.
Square law devices and half-wave and full-wave rectifier circuits process signals in a non-linear manner such that a DC term, related to the input signal level, is generated. However, for the half-wave and full-wave circuits, the output is not proportional to the input power level, and realizable square law devices are proportional only over a small dynamic range. Most non-linear approaches require some form of post-detection processing such as a look-up table where the detector output is used by the computer as an address to a memory input location where the value corresponding to the true (error corrected) input power level is stored. This approach is undesirable in that it requires computer time. In addition, these approaches require post detection filtering to prevent the AC input signal and its products, generated as a consequence of the non-linear processing, from affecting the desired output. The filter bandwidth must be small relative to the AC input frequency; this degrades the settling time performance.
Digital signal processing techniques may be used to compute the power of a waveform by sampling and quantizing the AC signal (usually a frequency translated version of the input signal to be measured) and computing the power based on the value of the digital samples. To minimize errors due to aliasing and quantizing, high speed, high resolution analog to digital (A/D) converters are required. The actual computation may be accomplished within the system computer. This is generally undesirable since the additional computational effort will significantly degrade system speed or require additional dedicated digital processing hardware further increasing the complexity of the system.
Synchronous detectors using video (baseband or DC) log conversion are exemplified by conventional network analyzers. These systems typically require the use of a phase locked loop. To achieve acceptable phase stability in the presence of noise, the loop filter would have to have an undesirably long time constant. This would increase the time to acquire lock on the input signal after a change in the frequency of the input signal and decreases system response time. Thus, such systems are typically too slow for frequency hopping applications.
In addition, the synchronous detection schemes typically employ a closed loop which must be stable over a wide input dynamic range. Wide variations in input signal level impact on the effective bandwidth of the system. This in turn adversely affects the noise performance of the system. To improve the noise performance of synchronous systems, limiters and AGC (automatic gain control) loops have been used.
Limiters clip the incoming AC signal and force it within the dynamic range of the system. AGC circuits scale the input signal down by a known amount bringing it within the dynamic range of the system. Both circuits increase system complexity. In addition, AGC circuits increase the settling time of the system.
Commercial detector log video amplifiers (DLVAs) operate without phase locked loops. The AC input signal is transformed into a DC output signal by multiple stages of limiting amplifiers. However, DLVA transfer functions exhibit discontinuities between stages. The discontinuities, ie., points of nonlinearity in the input-output response, adversely affect system accuracy and performance. The nonlinearities could be reduced with a computer look-up table, but this approach would be undesirable for the reasons mentioned above.
Since the nonlinearities are inversely proportional to the number of stages used, another solution would be to use additional stages. However, to achieve the degree of accuracy and linearity required in a frequency hopping antenna test and measurement system over a sufficiently wide dynamic range would require an impractically large number of cascaded stages.
Thermal techniques calculate power by converting the incoming waveform into thermal energy and measuring the temperature rise of the medium. Although accurate, this method is extremely slow and hence totally unsuitable for the present application.
Finally, conventional analog RMS and log converters use an analog log-integrate-antilog algorithm in operation. The major limitation of these systems is in settling time performance. The poor settling time performance is due to the relatively large time constant of the averaging integrator. This time constant must be sufficiently large (compared to the AC input frequency) to prevent the AC input signal from reaching the converter output. This ripple component would reduce the conversion accuracy and degrade the stability of the DC output to be measured. These systems are generally inadequate for the present application.
The related art thus demonstrates a need for a practical, stable, accurate, and flexible gain measurement system effective in the testing of an antenna in a frequency hopping mode. Such a system should be independent of the system computer and capable of quickly and easily switching between numerous input ports.